1. Chapter 9 Alkynes: An Introduction to Organic Synthesis

2. Learning Objectives Naming alkynes
Preparation of alkynes: Elimination reactions of dihalides
Reactions of alkynes: Addition of HX and X2
Hydration of alkynes
Reduction of alkynes
Oxidative cleavage of alkynes
Alkyne acidity: Formation of acetylide anions
Alkylation of acetylide anions
An introduction to organic synthesis

3. Alkynes Hydrocarbons that contain carbon–carbon triple bonds
Acetylene is the simplest alkyne
Polyynes - Linear carbon chains of sp-hybridized carbon atoms
Naming alkynes

4. Naming Alkynes General hydrocarbon rules apply
Suffix -yne indicates an alkyne
Number of the first alkyne carbon in the chain is used to indicate the position of the triple bond
Naming alkynes
Alkynyl groups are also possible

5. Worked Example Name the following alkynes: A) B)
2,5-Dimethyl-3-hexyne ,3-Dimethyl-1-butyne
Naming alkynes

6. Table: Priority of Functional Groups in Nomenclature
Compounds with more than one functional group.
Functional Group
Name as Main Group
Name as Substituent
Carboxylic acid
–oic acid
Carboxy
Ester
–oate
Alkoxycarbonyl
Amide
–amide
Amido
Nitrile
–nitrile
Cyano
Aldehyde
–al
Formyl
Ketone
–one
Oxo
Alcohol
–ol
Hydroxy
Thiol
–thiol
Thio
Amine
–amine
Amino
Alkene
–ene
Alkenyl
Alkyne
–yne
Alkynyl
Alkane
–ane
Alkyl
Ether
Alkoxy
Halides
Halo
Nitro

7. Preparation of Alkynes: Elimination Reactions of Dihalides
Treatment of a 1,2-dihaloalkane with KOH or NaOH produces a two-fold elimination of HX
Vicinal dihalides are available by addition of bromine or chlorine to an alkene
Intermediate is a vinyl halide
Preparation of alkynes: Elimination reactions of dihalides

8. Electronic Structure of Alkynes
Carbon–carbon triple bond results from sp hybrid orbitals of carbon forming a σ bond and unhybridized 2py and 2pz orbitals forming two π bonds
Remaining sp orbitals form bonds to other atoms at 180° from the C–C bond giving linear molecule
Breaking a π bond in acetylene (HCCH) requires 202 kJ/mole (in ethylene it is 269 kJ/mole)
Reactions of alkynes: Addition of HX and X2

9. Reactions of Alkynes: Addition of HX and X2
Addition reactions of electrophiles with alkynes are similar to those with alkenes
Regiochemistry according to Markovnikov’s rule
Reactions of alkynes: Addition of HX and X2

10. Reactions of Alkynes: Addition of HX and X2
Addition products are formed when bromine and chlorine are added
Initial addition gives trans intermediate
Intermediate alkene reacts further with excess reagent producing tetrahalide
Reactions of alkynes: Addition of HX and X2

11. Similarities in Alkene and Alkyne Additions
Mechanisms of alkene and alkyne additions reactions are similar but not identical
Addition of HX to an alkene is a two step process with an
alkyl carbocation as intermediate
Addition of HX to an alkyne is also a two step process with vinylic carbocation as the intermediate
Reactions of alkynes: Addition of HX and X2

12. Figure 9.2 - Vinylic Carbocation
Comprises an sp-hybridized carbon and forms less readily than an alkyl carbocation
Reactions of alkynes: Addition of HX and X2

13. Worked Example Identify the product of following reaction Solution:
Reactions of alkynes: Addition of HX and X2

14. Hydration of Alkynes Mercury (II)-catalyzed hydration of alkynes
Alkynes undergo hydration readily in the presence of mercury (II) sulfate as a Lewis acid catalyst
Reaction occurs with Markovnikov chemistry
Enol intermediate rearranges into a ketone through the process of keto–enol tautomerism
Hydration of alkynes

15. Keto-Enol Tautomerism
Enols rearrange to the isomeric ketone by the rapid transfer of a proton from the hydroxyl to the alkene carbon
Isomeric compounds that can rapidly interconvert by the movement of a proton are called tautomers
The keto form is usually so stable compared to the enol that only the keto form can be observed
Hydration of alkynes

16. Figure 9.3 - Mechanism of Mercury (II)-Catalyzed Hydration of Alkyne to Yield a Ketone
Addition of Hg(II) to alkyne gives a vinylic cation
Water adds and loses a proton
A proton from aqueous acid replaces Hg(II)
keto–enol tautomerism
Hydration of alkynes

17. Hydration of Unsymmetrical Alkynes
Hydration of an unsymmetrically substituted internal alkyne (RC≡CR’) results in a mixture of both possible ketones
Hydration of a terminal alkyne always gives the methyl ketone
Hydration of alkynes

18. Worked Examples What products are obtained by hydration of: Solution:
This symmetrical alkyne yields one product
Hydration of alkynes

19. Hydroboration-Oxidation of Alkynes
Borane adds to alkynes to give a vinylic borane
Oxidation with H2O2 produces an enol that converts to the ketone or aldehyde by tautomerization
Process converts alkyne to ketone or aldehyde with orientation opposite to mercuric ion catalyzed hydration
Hydration of alkynes

20. Comparison of Hydration of Terminal Alkynes
Mercury(II)-catalyzed hydration is Markovnikov and converts terminal alkynes to methyl ketones
Hydroboration-oxidation converts terminal alkynes to aldehydes because the addition is non-Markovnikov
Hydration of alkynes

21. Worked Examples
Name an alkyne that can be used in the preparation of the following compound by a hydroboration-oxidation reaction
Solution:
Hydration of alkynes. Phenylacetylene or Ethynylbenzene (IUPAC)

22. Reduction of Alkynes
Accomplished by addition of H2 over a metal catalyst (such as palladium on carbon, Pd/C). It is a two-step reaction
The addition of the first equivalent of H2 produces an alkene intermediate
The alkene immediately reacts with another equivalent of H2 and yields alkane
Reduction of alkynes

23. Reduction of Alkynes to cis-Alkenes
Addition of H2 using deactivated palladium on carbon catalyst (poisoned with traces of lead or CaCO3 and quinolone, called Lindlar catalyst) produces a cis alkene
The two hydrogens add syn (from the same side of the triple bond
Reduction of alkynes

24. Reduction of Alkynes to trans-Alkenes
Alkynes can also be converted to alkenes using sodium or lithium metal as the reducing agent in liquid ammonia as the solvent (-33 ºC), called
metal-ammonia catalyst.
This method produces trans alkenes
The reaction involves a radical anion intermediate
Reduction of alkynes

25. Mechanism of Li/NH3 Reduction of an Alkyne
Net result reduction of alkyne to trans alkene and formation of 2NaNH2

26. Worked Example Use any alkyne to prepare trans-2-Octene Solution:
Using the correct reducing agent results in a double bond with the desired geometry (cis or trans)
Reduction of alkynes

27. Oxidative Cleavage of Alkynes
Strong oxidizing reagents (O3 or KMnO4) cleave internal alkynes, producing two carboxylic acids
Terminal alkynes are oxidized to a carboxylic acid and carbon dioxide
The method was used to elucidate structures because the products indicate the structure of the alkyne such as internal or terminal
Oxidative cleavage of alkynes

28. Alkyne Acidity: Formation of Acetylide Anions
Terminal alkynes are weak Brønsted acids, pKa ~ 25 (alkenes, pKa ~ 44 and alkanes, pKa ~ 60, are much less acidic)
Reaction of a strong base with a terminal alkyne results in the removal of the terminal hydrogen and the formation of an acetylide anion
Alkyne acidity: Formation of acetylide anions

29. Acidity of Hydrocarbons
Methane and ethylene do not react with a common base
Acetylene can be deprotonated by any anhydrous base with a pKa higher than 25
Alkyne acidity: Formation of acetylide anions

30. Acidity is Based on the Percentage of s Character
Percentage of “s character” is based on the sp-hybridized carbon atom
Acetylide anions possess an sp-hybridized carbon with 50% s character
Vinylic anions have an sp2-hybridized carbon with 33% s character
Alkyl anions have an sp3-hybridized carbon with 25% s character
Alkyne acidity: Formation of acetylide anions

31. Comparison of Alkyl, Vinylic, and Acetylide Anions
Alkyne acidity: Formation of acetylide anions

32. Worked Example The pKa of acetone, CH3COCH3, is 19.3 Solution:
Which of the following bases with given solvents is strong enough to deprotonate acetone?
A) KOH/H2O (pKa of H2O = 15.7)
B) Na+ -C≡CH/NH3 (pKa of C2H2 = 25, NH3 = 36)
C) NaHCO3/H2O (pKa of H2CO3 = 6.4)
D) NaOCH3/CH3OH (pKa of CH3OH = 15.6)
Solution:
Na+ -C≡CH adequately deprotonates acetone
Alkyne acidity: Formation of acetylide anions

33. Alkylation of Acetylide Anions
Acetylide anion can react as nucleophile as well as base due to the negative charge and unshared electron pair on carbon
Reaction of acetylide anion with a primary alkyl halide produces a terminal alkyne providing a general route to larger alkynes. Further interaction of terminal alkyne with an alkyl halide gives an internal alkyne product
Any terminal alkyne can be used for making larger alkynes
Acetylide anion
Alkylation of acetylide anions
Alkynyl anion

34. Alkylation Reaction of Acetylide Anion
Alkylation of acetylide anions

35. Alkylation of Acetylide Ions
Acetylide ions cause elimination instead of substitution upon reaction with secondary and tertiary alkyl halides
Alkylation of acetylide anions

36. Worked Example Prepare cis-2-butene using propyne, an alkyl halide
Use any other reagents needed
Solution:
Hydrogenation of an alkyne, which can be synthesized by alkylation of a terminal alkyne, forms a cis bond
Alkylation of acetylide anions

37. Introduction to Organic Synthesis
Organic synthesis is vital to pharmaceutical industries, chemical industries, and academic laboratories
Planning a successful multistep synthetic sequence involves:
Utilizing available knowledge of chemical reactions
Organizing knowledge into a workable plan
Working in a retrosynthetic direction is important in planning an organic synthesis
Working backward
An introduction to organic synthesis

38. Worked Example
Use 4-Octyne as the only source of carbon in the synthesis of Butanal
Use any inorganic compounds necessary
Solution:
Butanal can be synthesized from either cis-4-octene or trans-4-octene
An introduction to organic synthesis

39. Worked Example
Synthesize decane using an alkyne and any alkyl halide needed
Solution:
Using retrosynthetic logic:
H2/Pd can reduce 1-decyne C8H17C≡CH to decane C10H22
Alkalylation of HC≡C:-Na+ by C8H17Br, 1-bromooctane produces C8H17C≡CH
Treatment of HC≡CH with NaNH2, NH3 gives HC≡C:-Na+
An introduction to organic synthesis

40. Summary
Alkynes are hydrocarbons comprising a carbon–carbon triple bond
Enols are formed when alkynes react with aqueous sulfuric acid in the presence of mercury (II) catalyst
Tautomerization is a process in which an enol yields a ketone
Reduction of alkynes can produce alkanes and alkenes
Acetylide anions are formed when a strong base removes alkyne hydrogen
In an alkylation reaction, an acetylide anion acts as a nucleophile and displaces a halide ion from a primary alkyl halide